Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: If using fresh Hatch chiles, roast them. You can do this under a broiler, on a grill, or over an open flame until the skins are charred and blistered. Place them immediately into a paper bag or a bowl covered with plastic wrap for 10-15 minutes to steam. This makes peeling easier. Once cooled, peel, stem, and seed the chiles, then dice them. If using canned, drain thoroughly and dice.
Step 3: Cook the elbow macaroni according to package directions until al dente. Drain well and set aside.
Step 4: In a large saucepan or Dutch oven, melt the butter over medium heat. Add the flour and whisk constantly for 1-2 minutes to create a roux. It should be light golden, like wet sand.
Step 5: Gradually whisk in the evaporated milk until smooth, ensuring no lumps. Continue to whisk and cook for 3-5 minutes, or until the sauce thickens and begins to simmer. Reduce heat to low.
Step 6: Remove the saucepan from the heat. Add the shredded cheddar and Monterey Jack cheeses, a handful at a time, stirring until each addition is fully melted and smooth before adding more. Stir in the diced Hatch chiles, garlic powder, onion powder, salt, and black pepper. Taste and adjust seasonings as needed. If you like a little extra heat, a pinch of cayenne pepper can be added here.
Step 7: Add the cooked macaroni to the cheese sauce and stir gently until all the pasta is evenly coated.
Step 8: Pour the macaroni and cheese mixture into the prepared baking dish. If desired, sprinkle a little extra cheese on top for a golden, bubbly crust.
Step 9: Bake for 25-30 minutes, or until bubbly and the top is lightly golden brown. Let it rest for 5-10 minutes before serving to allow the sauce to set slightly.